This invention relates to novel airbag coating compositions and systems comprising heat-expandable microspheres that provide effective insulation for the target airbag fabric during exothermic inflation. The inventive airbag fabrics are either pre-calendered prior to application of this composition or are coated through a gap (floating) knife method with such heat-expandable microsphere-containing compositions. The coated airbag fabrics may then be heated to expand the microsphere constituents of the coating compositions both within the interstices between the individual yarns of the fabric and over the raised yarns of the fabric. Such a coating system thus provides an extremely high degree of protection from heat exposure that permits structural integrity of the target airbag and provides protection from such high inflation temperatures to a vehicle passenger cushion during such a highly exothermic inflation event. The method of forming such specific airbag coating systems on airbag fabrics is also encompassed within this invention.
Airbags for motor vehicles are known and have been used for a substantial period of time. These devices are installed on the driver and passenger side of automobiles, as well as on the sides of both the front and rear compartments of vehicles (i.e., side airbags) and, in the event of a collision, are rapidly inflated with gas to act as an energy absorbing barrier between the driver or passenger and the steering wheel, dashboard, windows, or interior sides of the automobile. Such airbags (a.k.a., airbag cushions) must meet several important criteria. Included in this list are the ability to inflate quickly (within 0.1 seconds) after a collision event, the ability to rapidly deflate at a uniform rate (such as for front-seat airbags to provide a relatively soft cushion), the ability to retain inflation pressure for a relatively long duration (such as for at least 7 seconds for side airbags during rollover collisions), the ability to retain seam integrity over the target airbag fabric or cushion during and after an inflation event, and, possibly of greatest importance, the ability to withstand extremely high inflation pressures and temperatures during and after a collision event. This last issue concerns the utilized fabric""s structural integrity and thus its performance within an airbag cushion during a collision. Sodium azide has been the prominent inflation chemical utilized within standard airbag cushions for many years. In essence, upon collision, the inflation canister ignites the sodium azide (which is, in its pure form, highly toxic to humans) creating a small explosion forcing the released gas into the area of lowest pressure. In such an instance, the uninflated airbag cushion provides the escape route for such gas during the explosion. Thus, the airbag cushion inflates providing the protection for the driver or passenger as noted above. This entire inflation step occurs in about 0.1 seconds from the time of the collision event. In order to produce such a rapid inflation, the explosion produces a tremendous amount of heat within the inflation assembly, which also includes within the airbag cushion itself. Thus, the utilized airbag fabric must be able to withstand such large temperature variations and such exposure to heat without losing its ability to perform in its capacity to protect the driver or passengers.
There has been a recent movement away from sodium azide (due to its high toxicity, among other reasons) as the explosion ignition chemical utilized within airbag inflation assemblies. Although such a chemical may prove quite damaging to humans, it has, in the past, also provided a method of quick inflation for airbag cushions that required relatively low inflation temperatures (about 1200xc2x0 F.). New ignition chemicals, such as nitrocellulose-based compounds, have proven safer from a toxicity standpoint, but also produce extremely exothermic reactions. The use of nitrocellulose inflators (which do not need to be filtered as do the sodium azide-containing types) represents an improvement from a cost perspective over the sodium azide technology, but these unfortunately also produce even greater temperatures during deployment (about 2000xc2x0 F.).
In view of this movement away from sodium azide, modifications of such airbag fabrics and cushions are now necessary to protect the structural integrity of such fabrics and cushions during such an extremely high-temperature inflation event. Some thought has been given to utilizing higher denier yarns (i.e., 515, 630, and greater) to provide greater heat capacities for the target fabrics. However, such higher denier materials also create greater packed volumes for such target airbag cushions and increase the costs of manufacture for such airbag fabrics and cushions as to offset the savings provided through the use of a new inflation assembly. Thus, in order to benefit from the cost reductions associated with utilizing nitrocellulose inflator assemblies, there is a desire to utilize lower denier yarns (i.e., up to about 420 denier) with modified coatings to provide the necessary heat resistance. Such coatings must also provide the same performance standards as noted above for different airbag cushions (i.e., quick inflation, long-duration inflation, etc.). To date, there has been no discussion of altering prior well known airbag fabric coating compositions to compensate for this potential change from low-temperature sodium azide-containing inflation assemblies to those comprising chemicals which produce much higher temperature (about 2000xc2x0 F.) explosions and inflations.
In the past, coatings have been applied to fabrics intended for use in automotive airbags, both to resist the unwanted permeation of air through the fabric and to protect the fabric from the detrimental effects of the hot gases used to inflate the bags. Polychloroprene was the polymer of choice in the early development of such a product, but the desire to decrease the folded size of the completed airbag, and the tendency of poly(chloroprene) (a.k.a. neoprene) to degrade upon exposure to heat and release the components of hydrochloric acid (thereby potentially degrading the fabric component as well as releasing hazardous chemicals), has led to the acceptance of other compounds. Such compounds include silicone (polydimethylsiloxane, as merely one example), polyurethane, other rubber compositions, and the like. Such compositions have been well known as providing the desired permeability characteristics for such target airbag fabrics and cushions as well as temperature protection from the heat generated by sodium azide inflation explosions. However, and again, as noted above, there has been no teaching nor fair suggestion of any improvements in temperature protection for higher temperature, less expensive, and less toxic to humans, inflation chemicals within airbag inflation assemblies.
Although silicones and neoprene have been the predominant coatings utilized in the airbag industry traditionally, as noted above, it has been determined that these coatings exhibit certain shortcomings when exposed to the high temperatures associated with the unfiltered, non-sodium azide inflation assemblies. For example, since a complete coating (over the raised yarns of such airbag fabrics and within the interstices between such yarns) is necessary to effectuate the proper high temperature protection during an inflation event, large amounts of (expensive) silicones would be required. Also, low amounts of silicone elastomers do not provide the same heat resistance as thicker compounds. Thus, a composition of low amounts (cost-effective) of silicones but which completely coats the desired airbag fabric is necessary. Unfortunately, the prior art has not accorded the industry such a coating system. Neoprene degrades very easily and thus does not exhibit sufficient aging stability. Furthermore, very thick coatings of such rubber compounds are required to provide the complete coating as necessary with the increased inflation temperatures. These thick coatings result in much higher costs, which, when coupled with the lack of aging stability, makes neoprene alone a highly undesirable coating component. There clearly exists a need in the airbag coating art to provide a cost-effective, temperature-resistant coating system to meet these challenges.
It is thus an object of the invention to provide such a coating system. Another object of the invention has been to provide a cost-effective airbag coated fabric which can withstand the high temperatures associated with such new inflation assemblies. Yet another object of the invention is to provide an airbag cushion which will insulate a driver or passenger from the heat produced during an inflation event involving such new, unfiltered inflation assemblies. Accordingly, this invention encompasses an airbag fabric, at least a portion of which is coated with a coating composition, wherein said coating composition comprises heat-expandable microspheres, and at least one gas permeability reducing material. Furthermore, this invention encompasses a method of producing a coated airbag fabric comprising the steps of providing an uncoated fabric; coating at least a portion of said fabric with a composition comprising heat-expandable microspheres and at least one gas permeability reducing material, said coating step being performed by utilization of a floating knife technique prior to the heat-expansion of said heat-expandable microspheres; and exposing said coated fabric to a temperature sufficient to effectuate the expansion of said microspheres. Additionally, this invention encompasses a method of producing a coated airbag fabric comprising the steps of (a) providing an uncoated fabric; (b) calendering said fabric of step xe2x80x9caxe2x80x9d thereby flattening said fabric to a substantially uniform thickness with an average peak to valley distance between the interstitial space of the fabric and the apex of the raised yarns of the fabric of at most 10 microns; (c) coating at least a portion of said calendered fabric of step xe2x80x9caxe2x80x9d with a composition comprising heat-expandable microspheres and at least one gas permeability reducing material; and (d) heating the treated calendered fabric of step xe2x80x9ccxe2x80x9d to a temperature sufficient to effectuate the expansion of said heat-expandable microspheres. This invention also encompasses a method of producing a coated airbag fabric comprising the steps of (a) providing an uncoated fabric; (b) coating fabric of step xe2x80x9caxe2x80x9d with a solvent diluted composition consisting of heat-expandable microspheres and a gas permeability reducing material; said coating step performed on a gap (floating) knife coater where the coating blade height is greater than the heat-expandable microspheres"" diameter; (c) passing the coated fabric of step xe2x80x9cbxe2x80x9d through a heat zone sufficient to evaporate the solvent but not sufficient enough to cure the gas permeability reducing agent; (d) passing the coated fabric of step xe2x80x9ccxe2x80x9d through a second heat zone sufficient to expand the heat-expandable microspheres and cure the gas permeability reducing agent.
Such a specific fabric and such specific methods provide the necessary heat insulation and resistance and, when mixed with the gas permeability reducing material, permits a reduction in the amounts of such materials while still providing the requisite characteristics associated with airbag cushions. Basically, the expanded microspheres provide a barrier to high temperatures by not permitting the conduction of heat through the microsphere layer on the target fabric. Without sufficient surface to conduct the flow of heat from within the airbag cushion through to the outside surface, the high temperatures of the inflation event cannot be transferred to the fabric portion of the composite, thus allowing for the retention of the strength of the fabric.
The term heat-expandable microspheres is intended to mean any substantially spherical hollow objects (microballoons, as merely one example) which are at most 40 micrometers in diameter and which expand in average diameter size from about 1.2 to about 10 times their original size upon exposure to heat. Upon expansion, such microspheres should retain at least some of their spherical shape, although some objects may not uniformly increase in size to remain in such spherical configurations and some may also burst. Burst microspheres will still function properly within this invention since such materials will still provide a barrier of air between the gas permeability reducing material and the fabric surface (the air is trapped between such layers). Preferred microspheres for utilization within this invention are Expancel(copyright) (DU grade, for example, manufactured by Expancel Inc. of Duluth, Ga. To date, there has never been any utilization of heat-expandable microspheres with airbag fabrics or cushions at all, not to mention to provide the same heat insulation and resistance benefits as discussed above.
The term gas permeability reducing material basically encompasses any well-known airbag fabric or cushion coating material that provides a barrier, whether temporarily or permanently, to the gasses generated during an inflation event from moving from the interior of such an enclosure to the outside. Thus, such a material is selected from the group consisting of a silicone-containing compound, a polyurethane, a polyacrylate, a butyl rubber, EPDM, chloroprene (neoprene), a polyamide, an hydrogenated nitrile rubber, an ethylene-vinylacetate copolymer, and any mixtures or dispersions thereof. Silicone-containing materials are most preferred due to their excellent permeability reducing benefits as well as their aging stability. Such materials include, but are not intended as being limited to, polydimethylsiloxane and its derivatives (such as Dow Coming 3625 LSR silicone resin), and any mixtures thereof. From a cost perspective, the polydimethylsiloxanes are most preferred of this group. With regard to the other potential permeability reducing materials, potentially preferred materials include a polyurethane, available from Stahl USA, Peabody Mass., under the tradename Ru 40-350 (40% solids); polyacrylates, (a) available from Rohm and Haas, under the tradename Rhoplex(copyright) E-358 (60% solids), (b) available from Shell Chemical Company, Houston, Tex., under the tradename Epi-Rez(trademark) 5520 (60% solids), and (c) available from Para-chem Southern, Inc., Greenville, S.C., under the tradename Pyropoly AC 2000(trademark); a polyamide dispersion marketed under the trade designation MICROMID(trademark) 632 hpl by Union Camp Corporation which is believed to have a place of business in Wayne, N.J.; other polyurethane resins, Witcobond(trademark) 253 (35% solids), from CandK Witco, and Sancure, from BFGoodrich, Cleveland, Ohio; hydrogenated NBR, such as Chemisat(trademark) LCD-7335X (40% solids), from Goodyear Chemical, Akron, Ohio; and butyl rubber, such as Butyl rubber latex BL-100, from Lord Corporation. As noted above, mixtures or combinations of non-silicone materials may also be utilized such as a dispersion of polyurethane and polyacrylate, as merely an example. Potentially preferred compositions are noted below including dispersions comprising polyurethane and polyacrylate. Preferably, in such an instance, the ratio of polyurethane to polyacrylate should be in an amount of from about 0.1:1 to about 10:1; preferably from about 1:1 to about 8:1; more preferably from about 2:1 to about 5:1; and most preferably from about 2:1 to about 2.5:1. The add-on weight of this mixture of heat-expandable microspheres and permeability reducing material is from about 0.2 to about 2.0 ounces per square yard of the target fabric, preferably this add-on weight is from about 0.3 to about 1.5, most preferably about 0.8.
The substrate across which the cross-linked elastomeric resin coatings are applied to form the airbag base fabric in accordance with the present invention is preferably a plain woven fabric formed from yarns comprising polyamide, polyester, or blends of such fibers. Such yarn preferably has a linear density of about 100 denier to about 840 denier, preferably in the range of 210 denier to 420 denier. Such yarns are preferably formed from multiple filaments wherein the filaments have linear densities of about 6 denier per filaments or less and most preferably about 4 denier per filament or less. Such substrate fabrics are preferably woven using rapier looms or possibly through the utilization of fluid jet weaving machines as disclosed in U.S. Pat. Nos. 5,503,197 and 5,421,378 to Bower et al. (incorporated herein by reference). The fabric substrate with applied coating system will hereinafter be referred to as an airbag base fabric. Other possible components present within the microsphere-containing coating composition are solvents (such as water, volatile alcohols, ketones, aromatics and the like), thickeners, antioxidants, flame retardants, curing agents, coalescent agents, adhesion promoters, and colorants. Any well known thickener for airbag coatings (such as those comprising silicones, polyurethanes and/or polyacrylates) may be utilized in this invention. One potentially preferred thickener is marketed under the trade designation NATROSOL(trademark) 250 HHXR by the Aqualon division of Hercules Corporation which is believed to have a place of business at Wilmington, Del. Also, in order to meet Federal Motor Vehicle Safety Standard 302 flame retardant requirements for the automotive industry, a flame retardant is also preferably added to the compounded mix. Any well known airbag flame retardant may be used (including aluminum trihydrate, as merely one example). One potentially preferred flame retardant is DE-83R, 70% Dispersion marketed by Great Lakes Chemical.
It has been determined that the application of such microsphere-containing airbag coating compositions to the target airbag base fabrics in order to produce the desired heat insulation and resistance characteristics discussed above must be performed with specific procedures. Thus, once compounding is complete (and the preferably resultant dispersion possesses a viscosity of about 8,000-50,000 centipoise), the composition of microspheres and gas permeability reducing material may be applied in three different ways. First, the composition may be applied by any standard coating procedure such as, and not limited to, scrape coating. This term includes, and is not limited to, knife coating, in particular knife-over-gap table, gap (floating) knife, knife-over-roll, and knife-over-foam pad methods, to name a few different method types. However, in order to effectuate the desired coating on the target fabric surface, the fabric must first be calendered (flattened) such that the valley to peak distance of the interstitial spaces to the apex of the raised yarns is not greater than about 10 microns, preferably less than 8 microns, and most preferably less than 7 microns. The coated fabric may then be heated to a temperature sufficient to effectuate expansion of the microspheres. A second method involves the application of heat-expandable microspheres on uncalendered fabric using the gap (floating) knife technique. After application of the solvent-diluted composition consisting of the heat expandable microspheres and the gas permeability reducing agent, the coated fabric is heated to evaporate the solvent, but not sufficiently to cure the gas permeability reducing agent. If the fabric is overheated in this step and the gas permeability reducing agent is cured, the heat-expandable microspheres will not reach their full-inflated dimensions, as they are unable to expand against a cured matrix. In a subsequent step, the coated fabric is heated further to expand the microspheres and then cure the gas permeability reducing agent. In each method, the resultant airbag base fabric is substantially impermeable to air when measured according to ASTM Test D737, xe2x80x9cAir Permeability of Textile Fabrics,xe2x80x9d standards.
Such specific procedures must be followed in order to permit full coverage of both the interstitial spaces between the fabric yarns as well as the raised yarns of the same substrate. If the raised yarns are not sufficiently covered, the structural integrity of the fabric will most likely be compromised during the highly exothermic inflation event. Substantial weakening of the yarns at any location on the fabric will result in a loss of strength for the overall composite structure. Thus, the entire coated portion of the target fabric must be sufficiently protected by the insulating microspheres. Generally, the coating of a fabric with such microsphere-containing compositions would result in the movement (due to contact of the coating blade, cohesion, gravity, and other forces upon the microspheres themselves) of such a coating composition from the raised yam portions of the fabric into the interstitial spaces between such yarns. This problem is magnified when the peak to valley distance is too disparate (i.e., the slope angle of the raised yarns measured from the midline of the fabric to the apex of the raised yarns are relatively large) since the microspheres will easily xe2x80x9cslidexe2x80x9d into the interstitial spaces between the yarns in such an instance. This prevents sufficient contact of the microspheres with the raised yarns to impart the necessary heat insulation and resistance. It is important to note, again, that such microspheres are preferably heated to a temperature which does not result in bursting; however, even if such microspheres do not retain their structural integrity due to too great thermal expansion (and thus burst), since air will be trapped between the cured gas permeability reducing material and the fabric, the possibility of heat conduction remains low. Therefore, bursting of the microspheres is permitted and foreseeable within this invention.
Thus, the first method noted above initially produces a fabric with very low thickness and thus very little variation in thickness over the entire substrate. In such a manner, the slope angle and height of the raised yarns (corresponding to the midline of the fabric) are relatively low. As a result, the heat-expandable microspheres will not easily migrate into the interstitial spaces and will most likely remain over the raised yarns. The second method permits coverage of the fabric with a solvent diluted composition containing heat-expandable microspheres wherein the gap (floating) knife mechanism is set at a height above the target fabric greater than the thickness of the microsphere-containing composition. In such a manner, a thin coating may be applied evenly over the target fabric including the heat-expandable microspheres can fill the interstitial spaces as well as sufficiently cover the raised yarns.
As previously indicated, the substrate fabric is preferably a woven nylon material. In the most preferred embodiment such substrate fabric will be formed from fibers of nylon-6,6 woven on a rapier, water-jet, or air-jet loom. It has been found that such polyamide materials exhibit particularly good adhesion and maintenance of resistance to hydrolysis when used in combination with the coating according to the present invention.